The present invention relates generally to multi-ply composite metal products for cookware and, more particularly, to copper core, stainless steel clad composites for enhancing the thermal conductivity, corrosion resistance and cosmetic appearance in cookware products.
Stainless steel has long been recognized for its value as a corrosion resistant, highly ductile material in the manufacture of food preparation and handling equipment. Stainless steel is, however, a poor conductor of heat and many techniques have been conceived and developed to combine more heat conductive metals such as copper or aluminum with stainless steel. Copper is the most conductive of commercially available metals. A popular method of comparing the thermal conductivity of various metals is to assign copper a conductivity of 100%. By way of comparison, aluminum is approximately 55% the conductivity of copper and stainless steel possesses only 3% of the thermal conductivity of copper.
The direct bonding of stainless steel to copper is possible by several different methods, but each of these methods has serious drawbacks. The following is a brief review of these methods.
The first is the slab method which involves assembling thick plates of stainless steel and copper, first either by welding or explosion bonding. The bonded plates are then hot and cold rolled down to a usable gauge. The highest permissible hot working processing temperature for copper and lowest hot working processing temperature for stainless steel have a very small overlap. This leads to the necessity for multiple reheats and many steps in handling and processing. This technique is expensive and inherently has a low material yield, making it economically unfeasible for light gauges useful in the manufacture of cookware products such as pots and pans.
The second method of bonding is the cold reduction technique. Copper and stainless steel may be bonded by means of high rolling reduction at room temperature. However, the equipment to do this, in any other but very light gauges, would be extremely massive and is not available. The technique also requires a final anneal which once again raises the problem of dissimilar processing temperature ranges for stainless steel and copper.
The third method of bonding involves brazing. The direct brazing of copper and stainless steel is possible, but this practice requires the use of expensive silver based brazing agents, making the process economically unfeasible at this gauge. The resultant product also has some inherent forming limitations.
In pursuit of an economical combination of stainless steel with a conductive material, the cookware industry has adopted stainless steel bonded to one or both sides of aluminum. This material arrangement features ease of bonding and high product yield. The bonding process is done by heating and rolling and does not impose enough reduction in the stainless steel to require a post-bond anneal before forming. The processing temperature does not exceed 800.degree. F., which is well below the 1050.degree. F. at which stainless steel and aluminum begin to form an undesirable brittle intermetallic. Aluminum's lightweight and easy cold working make it a suitable material for the manufacture of high quality cookware. The aluminum core of such a product is usually an "Alclad" type product which is a prebonded strip of aluminum with a very thin skin of a higher purity aluminum (EC grade, or 1100, 1130, 1230, 1175, 7072, 1050, 1145, 1060 and like 1000 series alloys, etc.) on the exterior and an aluminum alloy core. The pure aluminum is needed in proximity to the stainless steel to achieve a suitable bond and the aluminum alloy core is present for its deep draw characteristic due to its higher strength. The aluminum alloy layer also prevents the "orange peel" phenomenon which would cause a rough texture on the exterior surface of the drawn product. Because of the nature of its grain, this can occur when a core of solid pure aluminum is used.
Despite its superior heat flow conductivity, the difficulties cited above have prevented copper from being used as a practical core material. U.S. Pat. No. 4,246,045 by John B. Ulam attempts to combine the desirable properties of a copper core with the ease of aluminum/stainless steel bonding. According to his work, the product is made by starting with an aluminum/copper/aluminum prebond sheet in which both aluminum components are of an Alclad type.
The copper cored prebond sheet is formed by cold reduction at room temperature. The '045 Ulam patent teaches that the presence of the Alclad aluminum prevents the occurrence of orange peel. The pure aluminum surfaces of the prebond core, according to the Ulam process, are cleaned and conditioned by chemical or abrasive means, then heated and bonded to stainless steel by methods similar to the bonding of aluminum cored stainless steel. The resulting 9-ply composite material is then deep drawn, by traditional means, into a cooking vessel.
In pursuit of a copper cored material for use in cookware, my work has led to close scrutiny of the '045 Ulam patent. I have found that important aspects of Ulam's teachings are contrary to the results of my experimental work.